Spiral Galaxies in Hickson 44
Images of clusters of galaxies never cease to inspire and amaze. The image below is of a relatively nearby cluster of galaxies called Hickson 44 and was taken by MASIL Astro Imaging with a 14.5-inch telescope, showing that you don’t need the space telescope to capture awe-inspiring images of the cosmos. Galaxies are distinguishable from stars in this image by the characteristic cross-like diffraction pattern produced by the point-source of stars. The individual stars, of course, are all within our own galaxy, the Milky Way (see below for a tangential anecdote about the Milky Way), which is a spiral galaxy like the three most prominent spiral galaxies toward the center of this image.
Image Credit and Copyright: MASIL Imaging Team. See also Astronomy Picture of the Day for a nice summary of the image.
The spiral structure of some galaxies (most other galaxies are “elliptical”, like the one in the upper left of this image) share the same basic physics of spiral density waves in Saturn’s rings. See here for an example of a spiral wave in the rings. The waves in the rings are wound up much more tightly than the spiral wave in a galaxy, which typically show only a couple of arms wrapped around only one or two times. In the case of the rings, the waves are caused by resonant interactions between ring particles and nearby moons. For galaxies, spiral waves can be produced by the irregular shape of the core of the galaxy, in effect acting sort-of like a giant moon interior to the arms of the galaxy.
So, the story with the Milky Way is this, and I think of it often to remind myself not to make assumptions about any class or group I’m addressing. I was talking to a fellow university student (many years ago) about galaxies and the structure of the universe, and I told the story of the origin of the name “Milky Way”. In Greek mythology, Hercules causes milk from his stepmother Hera’s (a god) breast to spill across the sky. When I told this story, the student looked at me in all seriousness and asked if I really believed that. This was at a mixer organized by the campus Baptist ministry, and the student was a member of that group. I was dumbstruck by the depth of his misconception of the scientific view of the universe.